PIPER PA60 DISTR PLT OVERSHOOTS ASSIGNED ALT SE OF EGC.

Narrative

DEPARTED MQI AT APPROX XA10 ENRTE TO OFP. CLBING VFR REQUESTED IFR CLRNC TO OFP; AND RECEIVED CLRNC TO CLB TO 5000 FT DIRECT EGC AS FILED. AS WE WERE CLBING; MONITORING INSTS I WAS CHATTING WITH THE OWNER OF THE ACFT AND SELECTED 5000 FT FOR THE AUTOPLT TO CAPTURE. I FULLY EXPECTED THE AUTOPLT TO CAPTURE THE ALT AT 5000 FT AND THEREFORE WAS CHKING NAV AND ENG INSTRUMENTATION. WHILE COMPLETING MY SCAN I REALIZED WE WERE AT 5400 FT AND BEGAN AN IMMEDIATE DSCNT TO 5000 FT AND WAS LEVEL AT 5000 FT IN LESS THAN 15 SECONDS. AT NO TIME DURING THIS DID ZDC CALL ANY TFC NOR DID WE SEE ANY TFC. I DO NOT FEEL AT ANY TIME WE WERE A CONFLICT OR THREAT TO ANY OTHER TFC BECAUSE OF THIS UNFORTUNATE ALTDEV. OBVIOUSLY A CLOSER WATCH OF INSTRUMENTATION WOULD HAVE PREVENTED THIS SIT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT HE HAD SET THE AUTOPLT TO CAPTURE AND THAT IT HAD ALWAYS CAPTURED THE SET ALT ON PREVIOUS OCCASIONS. IN THIS CASE WHILE HE WAS SOMEWHAT DISTRACTED BY A SINGLE PAX AND OWNER OF THE ACFT; THE AUTOPLT FAILED TO CAPTURE 5000 FT AND CONTINUED UNTIL NOTICED AT 5400 FT. THE ACFT WAS STARTED BACK TOWARD THE ASSIGNED ALT WHEN THE WASHINGTON CTLR QUIZZED THE RPTR ON ALT. THE RPTR ANSWERED 5400 FT DSNDING BACK TO 5000 FT. NO MORE DIALOGUE OF THE INCIDENT OCCURRED.